This invention relates generally to the printing industry and, in particular, to a bridge mandrel combining an inner metal tube and outer polymeric surface.
The Flexographic printing industry uses air-mounted thin sleeves to carry the printing plates in the press. Thicker sleeves, sometimes called xe2x80x9crepeat builders,xe2x80x9d can be manufactured to varying thicknesses (annular dimension) to economically provide a larger printing circumference which defines the repeat length.
An extension of this approach is xe2x80x9csleeve-on-sleevexe2x80x9d technology, wherein an intermediate xe2x80x9cbridge mandrelxe2x80x9d is used to provide the annular dimension or diameter on which a conventional or thin plate image-carrying sleeve can be mounted.
Until very recently, all sleeves, repeat builders and bridge mandrels were mounted on mandrels through the use of air pressure to expand the sleeve onto the mandrel by introducing high-pressure air in the annular space between the sleeve I.D. and the mandrel O.D. A soft compressible material (i.e., closed-cell urethane foam) is typically used to accommodate the expansion of the base fiberglass sleeve. However, this xe2x80x9csoftxe2x80x9d mount arrangement causes quality and durability problems. New press designs are offering hydraulic, hard-mount mechanical systems to firmly and more accurately secure bridge mandrels in place; the thin sleeves or repeat builders can then be air mounted onto bridge mandrels.
Mechanical or xe2x80x9chard mountxe2x80x9d bridge mandrels, particularly for print widths greater than 45xe2x80x3, require a tubular design to provide the necessary stiffness (deflection) without excessive weight. The industry is currently using bridge mandrels made from carbon fiber tubes to provide sufficient stiffness with minimum weight, but at considerable cost. There is an outstanding need, therefore, for a bridge mandrel which meets the stiffness requirements of this industry, while being less expensive than carbon-fiber products.
While it is generally held that thin-walled steel tubes cannot be used for repeat builders such as bridge mandrels due to wall-thickness variations and the extreme difficulties associated with machining, this invention overcomes the problem by applying a coating or layer of hard elastomer, epoxy, polymer or other suitable material to the OD surface of a thin-walled steel tube and machining this material to the necessary tolerances. The coating is chosen for a relative ease of machining combined with a resistance to thermal transfer. As such, the invention is able to utilize roll-formed and welded tubes using thin gauge steel sheet which can be economically formed, welded and transported in small quantities. The less accurate O.D., ovality and straightness associated with the roll formed and welded process is acceptable when the precision tolerances of a bridge mandrel are machined in the polymer coating and not in the steel tube.
As with existing carbon-fiber and other bridge mandrel designs, the mandrel according to this invention is adapted for mounting on the outer surface of a shaft or roller of the type use in the Flexographic printing process. An end cap is disposed at each end of the tube, each end cap having a central bore to receive the shaft or roller. The outer layer to be machined is bonded directly or indirectly to the metal tube, thereby defining a precise, desired final diameter upon which to mount the printing sleeve.
Although in the preferred embodiment the thin-walled, cylindrical metal tube is a type of steel, aluminum and various metal or composite alloys may alternatively be used. A cavity is created between the outer surface of the shaft or roller and the inner surface of the metal tube, and at least one of the end caps preferably includes a passageway from the cavity through the tube to the outer surface, such that the cavity may be pressurized to assist in installing and removing the outer sleeve, as is common practice in the Flexo industry. To accommodate existing plates, the length of the mandrel may be 96 inches or less, with a circumference of perhaps 36 inches or less, and a wall thickness of 0.125 inches or less. However, these preferred dimensions are based upon current technology and are therefore subject to change in accordance with the invention as lengths and diameters increase, as anticipated.
The polymeric coating is preferably a hard polyurethane, though other materials may be used, including natural or synthetic rubber or other elastomers, or ABS, PVC, acrylics or other plastics or epoxy-based layers. In some cases, a fiber-reinforced layer may be disposed between the outer surface of the metal tube and the polymeric layer. At least some of the fibers may be oriented longitudinally with respect to the metal tube, or circumferentially, to enhance bending and/or hoop strength, respectively. The fiber-reinforced layer may include a type fiber glass or aramid fibers, or alternative materials, along with appropriate bonding resin.